A known example of air bag system for a motor vehicle, in particular, an air bag system for the passenger seat, is illustrated in FIG. 8. As shown in FIG. 8, a lid 3 is provided for closing an opening 2 formed through an instrument panel 1 on the side of the passenger seat in the vehicle compartment, and an air bag module 4 is stored inside the lid 3.
The air bag module 4 is principally composed of the lid 3, an air bag 5, an inflator 6, and a reaction can 7. Inside the instrument panel 1, the air bag module 4 is mounted on a vehicle body such that the reaction can 7 is fixed to a deck cross pipe 9 which is a constituent member of the vehicle body.
The inflator 6 is adapted to generate high-pressure gas in response to a signal from an ECU (not shown), and the air bag 5 that is normally folded in the opening portion of the reaction can 7 is caused to open the lid 3 and deploy toward the passenger seated in the vehicle compartment, utilizing the gas generated by the inflator 6.
An impact detection sensor is connected to the ECU, which generates a signal to cause the inflator 6 to generate high-pressure gas upon detection of an impact of not smaller than a predetermined or threshold value. The air bag 5 deploys when an impact equal to or greater than the predetermined value is applied to the vehicle, and starts contacting with the upper part of the body of the passenger at the point of time when the air bag 5 deploys to the full extent. The air bag 5 then restrains the upper part of the body while gradually contracting with the gas being discharged through a vent hole 5a provided at a side portion of the air bag 5 for controlling the pressure within the air bag, thereby to alleviate or reduce the impact applied to the passenger.
Where the passenger is seated close to the instrument panel, however, the air bag comes into contact with the passenger before it deploys to the full extent, and the impact applied to the passenger becomes large since the air bag deploys to the full extent after contacting with the passenger. In such a case, the air bag cannot satisfactorily provide an intended or desired effect.
In a known air bag system as disclosed in Japanese laid-open Patent Publication (Kokai) No. 4-92738, where the vehicle collides with an obstacle while running at a speed equal to or lower than a predetermined speed, the air bag module as a whole is moved further into the inside of the instrument panel (in the traveling direction of the vehicle), so that the size of the air bag that deploys outwardly of the instrument panel is reduced to be smaller than that in normal use. With this technology disclosed in the above-identified publication, however, the magnitude of deployment of the air bag into the vehicle compartment upon actuation of the air bag can be reduced to be smaller than that in normal use only when the collision occurs at a vehicle speed equal to or lower than the predetermined speed, and the above-described problem that occurs when the passenger is seated close to the instrument panel cannot be solved. Furthermore, a glove box, and the like, are provided in the vicinity of the air bag within the instrument panel, leaving no extra space, and it is therefore difficult to move the whole air bag module further into the inside of the instrument panel.